The present invention relates to a large-sized substrate suitable for a synthetic quartz glass substrate for photomask, particularly a substrate for use in a TFT liquid crystal panel, and a method of producing the same.
Generally, for a TFT liquid crystal panel, an active method is adopted in which a liquid crystal is sealed between an array-side substrate with TFT devices incorporated therein and a substrate fitted with a color filter, and voltage is controlled by the TFT to thereby control the orientation of the liquid crystal.
At the time of producing the array side, a method is adopted in which images are printed on a mother glass such as non-alkali mother glass in a multiple layers by light exposure through originals carrying circuits drawn thereon, called large-sized photomasks. On the other hand, the color filter side is also produced by a method using lithography, called a dye impregnation method. The large-sized photomasks are required for production of both the array side and the color filter side, and, for carrying out light exposure with high accuracy, a synthetic quartz glass with a low coefficient of linear expansion is primarily used as the material of the large-sized photomasks.
Hitherto, enhancement of precision of the liquid crystal panel has been advanced from VGA through SVGA, XGA, SXGA, and UXGA to QXGA, and it is said that a precision of 100 ppi (pixels per inch) class to 200 ppi class is necessary. Attendant on this, the requirement for the exposure accuracy on the TFT array side, particularly the register accuracy has come to be more and more rigorous.
In addition, production of the panel by the so-called low temperature polysilicon technology has also been conducted. In this case, it has been investigated to print a driver circuit and the like on an outer peripheral portion of a glass, separately from the pixels of the panel, and light exposure with a higher precision has come to be demanded.
On the other hand, as to the substrate for large-sized photomask, it is known that its shape influences the exposure accuracy. For example, where exposure is conducted by use of two large-sized photomask substrates differing in flatness, as shown in FIG. 1, the pattern would be staggered due to the difference in optical path. Namely, in FIGS. 1A and 1B, the broken lines show the optical paths in the case where the rays go straight and the mask has an ideal plain surface, but, actually, the rays are deviated as indicated by the solid lines. In addition, in the case of an exposure machine which uses an optical system having a focus, there is the phenomenon that the focus position is staggered from the exposure surface with the result of poor resolution. Therefore, a large-sized photomask substrate having a high flatness is desired, for achieving exposure with a higher accuracy.
Besides, for the purpose of obtaining a multiplicity of exposed patterns by a single exposure and enhancing the productivity of the panel, a large sized photomask substrate of, for example, 1500 mm in diagonal length has come to be demanded. Thus, a large size and a high flatness are demanded at the same time.
In general, a large-sized photomask substrate is produced by a method in which a plate form synthetic quartz is lapped by use of a slurry including free abrasive grains such as alumina suspended in water, to remove ruggedness of the surface of the plate form synthetic quartz, and thereafter the surface is polished by use of a slurry including an abrasive such as cerium dioxide suspended in water. The processing device used in this case is a double side processing machine, a single side processing machine or the like.
However, these processing methods have had the following drawback. In these methods, the repelling force against the elastic deformation generated when the substrate itself is pressed against the processing surface plate is utilized for correction of flatness. Therefore, when the substrate size is enlarged, the repelling force is lowered conspicuously, so that the capability of removing gradual ruggedness of the substrate surface is lowered. FIG. 2A shows the shape of the substrate 1 when the substrate 1 is held vertically, and FIG. 2B shows the shape of the substrate 1 during processing, illustrating that the substrate 1 is conforming in shape to the surface plate during processing. FIG. 2C illustrates the repelling force against the elastic deformation of the substrate 1 during processing; thus, the portion corresponding to the repelling force is processed more than the other portions are, by an amount (ΔP) corresponding to the force.
Generally, a plain surface grinder adopts a method in which the work is passed through a fixed spacing between a work mount table and a processing tool, whereby the portions of the work in excess of the fixed spacing are removed by the processing tool. In this case, the work is pressed against the work mount table due to the grinding resistance of the processing tool; therefore, if the flatness of the back side of the work is not secured, the flatness of the face side of the work processed conforms to the flatness of the back side, so that the flatness cannot be improved.
Accordingly, in the case of a large-sized photomask substrate, it is very difficult to obtain a high flatness, although it is easy to suppress the dispersion of thickness of the substrate. Thus, the flatness of the substrate obtained by the prior art, in terms of the ratio of flatness/diagonal length, has been about 10×10−6 at best, though depending on the substrate size.
Therefore, the flatness of the large-sized photomask substrate for TFT exposure currently available, say, in the case of a substrate 330×450 mm in size, is limited to 4 μm and the flatness/diagonal is limited to 7.3×10−6; even for more larger substrates, a flatness/diagonal value lower than 7.3×10−6 is absent under existing conditions.
In the conventional lapping processing, the repelling force against the elastic deformation of the substrate during processing is primarily utilized for correction of flatness, as has been mentioned above, so that a substrate with poor flatness tends to be improved in flatness in a comparatively short time. However, as the flatness is improved, the elastic deformation amount becomes smaller and, hence, the repelling force also becomes smaller, so that the flatness would not easily be further enhanced. In such a case, in practice, only the processing margin is increased, and it has been impossible to obtain a substrate with high flatness by the conventional lapping. This problem is present also in the case of plain surface grinding.